The invention relates to a live deletion mutant of a pseudorabies virus (PRV), also called Aujesky's disease virus (ADV). Pseudorabies is a disease of all domestic animals with the exception of the horse, and causes severe damage, especially among swine and cattle. The pig is the natural host of the herpes virus of Aujesky's disease. In pigs, PRV infection may cause respiratory illness and encephalitis, which can ultimately lead to death.
Animals are infected with PRV via a nasal route. After initial virus multiplication in the mucous membranes of the upper respiratory and digestive tracts, the virus spreads via nerves to the brain. The severity of the infection may vary from acute to sub-clinical, being mainly dependent on the virulence of the virus and the age of the infected animal. Like other herpes virus, PRV induces latent infections, particularly in nerve tissue.
Vaccination against Aujesky's disease is carried out to limit the economic damage caused by mortality and growth retardation in infected animals. For this purpose, vaccines based on attenuated live virus and on inactivated virus are available. Attenuated live virus vaccines are generally preferred, since they are more easily produced and therefore less expensive than inactivated virus. Moreover, attenuated live virus vaccines can be administered intranasally which provides better protection than parenteral vaccination with either attenuated live virus or inactivated virus.
Early vaccines based upon attenuated live virus strains possessed several disadvantages. For example, these vaccine strains were generally produced by serially passaging virulent or mildly virulent strains in tissue culture (50-900 passages), which induced uncontrolled mutation in the virus. As a result, the compositions of such vaccines were not homogeneous and viral variants of unknown virulence and vaccinating power were included in the mixtures. Moreover, such vaccines suffered from a risk of reverting to virulence which could induce latent infections.
The development of techniques for controlled manipulation of genetic material has allowed the possibility of obtaining attenuated live virus vaccines which avoid these disadvantages. The structure of the PRV genome has been described in the literature (Virology 97, 151-163 (1979)). The PRV genome contains about 160,000 nucleotide pairs containing two inverted repeats and two unique sequences, one small and one large, designated U.sub.s and U.sub.1, respectively. On the basis of the arrangement of its reiterated DNA sequence, PRV has been classified as a D-herpes virus.
Various studies also reported that a functional thymidine kinase gene was required for the development of undesirable latent infections. Thymidine kinase deficiency was therefore used as a marker in the selection of useful vaccine strains. Subsequently, Kit et al. described virus strains formed by selective mutation in the thymidine kinase gene, including the formation of deletion mutants. (U.S. Pat. Nos. 4,514,497 and 4,609,548 to Kit et al.)
U.S. Pat. No. 4,680,176 to Berns et al., which is incorporated herein by reference, describes vaccine strains of PRV showing reduced virulence which contain either or both of two deletion mutations in two selected regions of the genome. The especially preferred strain of Berns et al. is strain 2.4N3A which contains an approximately 100 bp deletion in an inverted repeat sequence and a 2.4 Kbp deletion in U.sub.s. Other vaccine strains of Berns et al. may additionally include a deletion in the thymidine kinase gene, preferably of 100-500 base pairs (bp) in size.